[llvm-dev] Adding [[nodiscard]] to Compiler.h

[Justin Bogner via llvm-dev]

Justin Bogner <mail at justinbogner.com> writes:
> Chandler Carruth <chandlerc at google.com> writes:
>> My 2 cents: get rid of LLVM_UNUSED_RESULT, and move to LLVM_NODISCARD.
>>
>> For compilers that support it, it should be a strict superset of features and
>> functionality. The standard feature was written directly based on the clang
>> warn_unused_result stuff.
>>
>> I would just migrate us onto the spelling and usage pattern that got
>> standardized. All we have to lose are warnings from compilers other than Clang
>> until they implement this feature. That seems like not a huge loss to me
>> honestly.
>
> You might be right. It seems a bit unfortunate that we'd lose these
> warnings on MSVC and gcc for the time being, but maybe that's better
> than the usability cost of having two macros that expand to the same
> thing.
>
> Do note though, if we do that this would mean clang is the only compiler
> we can support this on at all without -std=c++17, since [[nodiscard]]
> will trigger extension warnings in earlier standard modes.

After talking to a few people it sounds like one LLVM_NODISCARD macro is
the way to go - We have prior art for clang-only warnings; we have
enough coverage building with clang that we'll still catch anything
quickly; and having two macros that are so similar is confusing and
difficult to use.

I'll stage this as adding an LLVM_NODISCARD, converting in tree code to
use it instead of LLVM_ATTRIBUTE_UNUSED_RESULT, then finally removing
the old macro.